1. Field of the Invention
The present invention relates to a base station apparatus, communication terminal apparatus, communication system, and communication method that perform multicarrier communication.
2. Description of the Related Art
In recent years, various kinds of information apart from voice, such as images and data, have come to be transmitted in radio communications, and particularly in mobile communications. With the demand for the transmission of various kinds of content expected to continue to grow in the future, an increasing need for highly reliable, large-capacity, high-speed transmission is also anticipated. When high-speed transmission is performed in mobile communications, the effects of delayed waves due to multipath propagation cannot be ignored, and transmission characteristics degrade due to frequency selective fading.
Consequently, multicarrier modulation methods such as OFDM (Orthogonal Frequency Division Multiplexing) are attracting attention as one kind of technology for combating frequency selective fading. A multicarrier modulation method is a technology that achieves high-speed transmission by transmitting data using a plurality of carriers (subcarriers) whose transmission speed is suppressed to a level at which frequency selective fading does not occur. With the OFDM method, in particular, the subcarriers on which data is placed are mutually orthogonal, making this the multicarrier modulation method offering the highest spectral efficiency. Moreover, the OFDM method can be implemented with a comparatively simple hardware configuration. For these reasons, OFDM is an object of particular attention.
In mobile communications, a so-called multi-cell method is employed whereby an extensive communication area is provided by dividing the communication area into a plurality of planar zones called cells and deploying a base station apparatus (hereinafter also referred to as BS: Base Station) for each cell. A mobile communication apparatus (hereinafter also referred to as MS: Mobile Station) must perform a process called a cell search to select a BS with low transmission loss that is the most suitable for communication among a plurality of BSs in a multi-cell environment.
OFDM cell search methods have been disclosed, for example, in Patent Document 1 (Unexamined Japanese Patent Publication No. 2002-335557) and Patent Document 2 (Unexamined Japanese Patent Publication No. 2003-264524). Also, an OFDM modulation method has been disclosed in Patent Document 3 (Unexamined Japanese Patent Publication No. HEI 8-97798).
According to the communication apparatus description in Patent Document 1, a control channel area and data channel area are divided in the time-axis direction, a frequency in the control channel area is divided into a subcarrier group, each BS transmits a cell search channel assigned to a divided subcarrier group, and an MS measures the received field strength of a received cell search channel and determines if it is in a state in which communication with a BS is possible.
According to the description of a mobile station and base station apparatus in Patent Document 2, a data channel area and control channel area are divided into subcarriers, each BS transmits a control channel assigned to a specific subcarrier, and an MS receives only a subcarrier group to which a control channel is assigned and selects the optimal BS for communication based on control channel information.
Patent Document 3 discloses an OFDM modulator that reduces multipath interference by applying the properties of even-numbered and odd-numbered subcarriers in the OFDM modulation method.
FIG. 1 is a drawing showing the frequencies and frame configuration of a conventional OFDM system. In FIG. 1, the vertical axis indicates time and the horizontal axis indicates frequency. To be more specific, time on the vertical axis is shown by frame timings 1, 2, . . . , and frequency on the horizontal axis is shown by OFDM system subcarrier numbers 1, 2, . . . . Hatching in FIG. 1 indicates data.
As shown in FIG. 1, in a conventional OFDM system, in addition to the necessity of a null symbol 11 for detecting frame timing, a cell search channel 12 is also necessary for identifying a base station to be communicated with after null symbol 11 is detected, and thus one frame includes two OFDM symbols that are not communication data.
However, there are problems with this kind of conventional cell search method, as follows.
In the case of an apparatus described in Patent Document 1, a cell search channel itself does not include a means of differentiating itself from another symbol in the time-axis direction, and a special symbol is provided, separate from the cell search channel for received field strength measurement, that differentiates between a control channel area and a data channel area in the time-axis direction in order to identify the position of a cell search channel in the time-axis direction. There is thus a problem of the data part transmission amount being reduced.
In the case of an apparatus described in Patent Document 2, the BS to be communicated with is determined according to the size of the power average value of a control channel assigned to a specific subcarrier, and there is a problem of not being able to track fluctuations in subcarrier received power due to frequency selective fading.
In the case of an apparatus described in Patent Document 3, although the properties of even-numbered and odd-numbered subcarriers in OFDM are applied, it can hardly be said that the degree of loss in BS-MS communications is faithfully reflected, and furthermore time-axis direction timing identification is not mentioned at all.